American Scientist

To the Editors:

I enjoyed Henry Petroski’s column on the evolution of the pill organizer (“Pill Organizers as Organized Design,” Engineering, January–February). Petroski notes that the thinned-out section of injection-molded plastic at the rear of the organizer serves as a hinge for the compartment covers, and that these hinges are prone to fatigue failure over time.

Mechanical engineers and plastics folk refer to such hinges as living hinges, in which a material’s elastic and plastic properties allow for captive movement, rather than having two or more rigid parts that can rotate relative to one another and are connected with a hinge pin or similar pivot.

Polypropylene is well known for its ability to serve this purpose because of its long cycle life. Well-designed living hinges can survive hundreds of thousands to millions of cycles.

The plastic hinges of my pill organizer, which I have used for several years with no failure, show a combination of elastic and plastic properties. When I first pop open the lids, they stay in the near-closed position, but I can bend them back, after which they stay nearly vertical, which makes it easier to fill the compartments.

Doug Ely
North Andover, MA